A professor visited a Zen master to enquire about Zen. As the master was speaking the professor kept interupting with his own opinions. So the master served some tea. He overfilled the cup and tea went everywhere. The professor shouted "the cup is full, there is no room for more tea!" The master replied "like this cup, your mind is so full of its own opinions, there is no room for anything new, in order to taste my tea, you must first empty your cup."